i've made quite a lot of tweaks particularily for when the panels are overlayed onto the artist picture.
For the picture overlay setting, when the opacity is set < 0.6 it fills the right hand panel with semi-transparent black so you can see the text better. Its still not ideal for low opacity settings but i am not planning to change that.
IMHO,it doesn't look really good. It was really better with the previous patch.
About the lyrics, ATM, MusicBee apply a transparency to the text. That's why it's not really readable in some cases. It's not 'cause of the overlay opacity.
So, something like this would be better:
layer #1: background (fanart) -> opacity=100%
layer #2: overlay -> opacity=0-80% (can be set in option)
layer #3: album cover, track info, lyrics and visualiser (without background) -> opacity=100%
layer #4: player bar, MusicBee icon and menu, Windows close, restore, etc. buttons
So, dunno if the lyrics are on the layer #2 or if you apply the transparency setting on the overlay and the lyrics, but I think something like this would be better:
1st picture: previous patch, opacity=40% / 2nd picture: last patch, opacity=60% / 3rd picture: how I think it should look like

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As you can see on the third picture, lyrics are readable even without overlay. Just added a shadow effect to the fonts (since border effect doesn't look good in MusicBee). So, you should try to get ride of this transparency on the lyrics. Just my 2 cents